I bought a basic filament runout sensor, but I didn't get the mounting plate for it, so I redesigned an existing model by jd94 on Thingiverse:
https://www.thingiverse.com/thing:4758845
No additional mount is required, you can simply screw it to the Z axis and it will fit next to the extruder (I used flat screw to be sure). You can use longer computer case screws for mounting the sensor.
Infill: I recommend at least 25-30%, to get some rigidity.
Support: I was able to print it without supports on my Ender 3.
The author remixed this model.